The role of mild and hard anodization regimes of iron oxide nanotubes in the photoelectrochemical performance

Abstract

• Fe 2 O 3 nanotubes (NTs) prepared by electrochemical anodization for solar water splitting applications. • Two regimes of anodization, Mild and Hard, observed in NTs geometrical features ( D p , D int , W , L ) and anodization curves ( j ( t ) and Q ( t )) • Relation between charge curves Q ( t ), nanotubes length ( L ), and introducing porosity ( P ) as a parametric, established to estimate L by an adapted Classical Faraday’s Law. • Estimated L from adapted Classical Faradaýs Law in excellent agreement with the experimental data from SEM. Self-ordered hematite (α-Fe 2 O 3 ) nanotubes have been intensively studied as the ideal geometrical configuration for photoelectrochemical water splitting. In this work, iron oxide nanotubes were produced by a fast electrochemical anodization at different applied voltages (10 to 100 V) and studied their impact on the photoelectrochemical performance of the photoelectrodes. The anodization voltage highly affects the nanotubes length, pore diameter, interpore distance and porosity that ultimately led to different solar performances. The interplay between oxidation and dissolution rates in the electrochemical process at different voltages led to observe two distinct self-ordering NTs growth regimes: (I) mild-anodization regime from 20-60 V, where both NTs linear growth and geometrical structural ordering are observed; (II) hard-anodization regime from 70-100 V, with non-linear decay of nanotubes thickness combined with a decrease of structural ordering. The highest photocurrent of ∼0.5 mA·cm -2 was obtained for sample anodized at 20 V with a porosity of 30 %. A relation between the anodization charge curves Q ( t ), nanotubes length ( L ) and porosity ( P ) was established to estimate L using Q ( t ) curves. L was found to be in excellent agreement with the experimental data from both scanning electron microscopy (SEM) and anodization curves.